I had about 40 kg of lead which I melted into a diamondplate box I made and bolted that into the frame I made for the transport hack, so it was under the flat floor.
I now have a "lastenseitenwagen" as hack and just bolted it to the floor, since it's pretty flat it doesn't take too much space.
This is next to my MZ 301 2-stroke.
On my R 90/6 with SuperElastik sidecar I have about the same weight in the boot, as far to the outside as possible, mounted with a bolt with a wingnut so I can take it out pretty easy. Just have to make some kind of grip on it so I can pick it easier